The Live-A-Little e-ink display updates with a new "years remaining" prediction when the solar panel on the back of the device receives direct sunlight for at least 20 seconds. Use the suction cups to secure it to a window that sees direct sunlight for at least part of the day. The device will automatically update several times on sunny days and not at all on cloudy days.
If you are not particularly fond of the current prediction, pressing the button on the front of the display while the device is in sunlight will also update the display. Updating your current age simply requires using a jumper wire with the pins on the back of the device while in sunlight. Pin functions are "Set", "+", and "-".
All of this "while in direct sunlight" business means that the Live-A-Little device is fully solar powered and does not require batteries.
If you have access to a 3D printer and soldering iron, the total cost to build a Live-A-Little is less than $30.